Drive train - backlash(?)

Good evening,

Today I experienced a random, brief vibration whilst driving, seeming to come from the rear with a ‘graunching’ noise. It did it once and then on the way home, didn’t again. Looking under the Jimny once home, there was nothing obvious, though when first reversing, there was a knock. Shifting back to first the knock came again, though it is only once when moving between forward and reverse. Whilst under the car I noticed backlash in the rear prop. I wonder if this is normal, perhaps the source of the knocking and perhaps the vibration? Looking for some advice if possible. I’m concerned it is the start of something bad and would like to catch it before it worsens.
Thanks! 

There will always be backlash and rotational play when you turn the drive shaft. It is the sum of all backlash in the gear mesh and must be there to ensure an optimal contact pattern between the gears.
My clunck noise have been getting bigger or more lately but then my car is abused and wear out faster.

The best way to check the diff is to take it out. But the easiest is to Jack up one rear wheel and, with the car in neutral 2wd, parking brake off, turn the wheel by hand first one way then the other, and feel for bumps or noises. Then the other wheel, same.
then both off the ground, in gear, engine off. When turning one wheel, the other one should go the opposite way smoothly.